It's explained in chapter 17 and 18 (Q and R)
The school massacre was a murder + arson and the girl who was feeding photos to fish wasn't actually the survivor but just someone who looked like her (probably crazy or trolling). The rest was added by Fukushima.
Ah, I see. When I first read it, I assumed Fukushima was just theorizing or lying. When Hiruma listened to the tape and started bleeding and freaking out, I thought something supernatural was happening, but it was simply the sound of a classroom being massacred, and he reacted appropriately.
